Title: Oxidative Hydrophenylation of Ethylene Using a Cationic Ru(II) Catalyst: Styrene Production with Ethylene as the Oxidant

Abstract

The complex [(MeOTTM)Ru(P(OCH 2) 3CEt)(NCMe)Ph][BAr' 4] (MeOTMM = 4,4',4"-(methoxymethanetriyl)-tris(1-benzyl-1H-1,2,3-triazole), BAr' 4 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) is used to catalyze the hydrophenylation of ethylene to produce styrene and ethylbenzene. The selectivity of styrene versus ethylbenzene varies as a function of ethylene pressure, and replacing the MeOTTM ligand with tris(1-phenyl-1H-1,2,3-triazol-4-yl)methanol reduces the selectivity toward styrene. For styrene production ethylene serves as the oxidant to produce ethane, as determined by both 1H NMR spectroscopy and GC-MS. The Ru(III/II) potentials of [(MeOTTM)Ru[P(OCH 2) 3CEt](NCMe)Ph][BAr' 4] (0.86 V) and [(HC(pz 5) 3)Ru[P(OCH 2) 3CEt](NCMe)Ph][BAr' 4] (0.82 V) (HC(pz 5) 3 = tris(5-methyl-pyrazolyl)methane) are nearly identical. Since catalytic conversion of ethylene and benzene by [(HC(pz 5) 3)Ru[P(OCH 2) 3CEt](NCMe)Ph][BAr' 4] is known to selectively produce ethylbenzene, the formation of styrene using [(MeOTTM)Ru[P(OCH 2) 3CEt](NCMe)Ph][BAr' 4] is attributed to the substituents on the triazole rings of the MeOTTM ligand.

@article{osti_1596958,
title = {Oxidative Hydrophenylation of Ethylene Using a Cationic Ru(II) Catalyst: Styrene Production with Ethylene as the Oxidant},
author = {Jia, Xiaofan and Gary, J. Brannon and Gu, Shaojin and Cundari, Thomas R. and Gunnoe, T. Brent},
abstractNote = {The complex [(MeOTTM)Ru(P(OCH2)3CEt)(NCMe)Ph][BAr'4] (MeOTMM = 4,4',4"-(methoxymethanetriyl)-tris(1-benzyl-1H-1,2,3-triazole), BAr'4 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) is used to catalyze the hydrophenylation of ethylene to produce styrene and ethylbenzene. The selectivity of styrene versus ethylbenzene varies as a function of ethylene pressure, and replacing the MeOTTM ligand with tris(1-phenyl-1H-1,2,3-triazol-4-yl)methanol reduces the selectivity toward styrene. For styrene production ethylene serves as the oxidant to produce ethane, as determined by both 1H NMR spectroscopy and GC-MS. The Ru(III/II) potentials of [(MeOTTM)Ru[P(OCH2)3CEt](NCMe)Ph][BAr'4] (0.86 V) and [(HC(pz5)3)Ru[P(OCH2)3CEt](NCMe)Ph][BAr'4] (0.82 V) (HC(pz5)3 = tris(5-methyl-pyrazolyl)methane) are nearly identical. Since catalytic conversion of ethylene and benzene by [(HC(pz5)3)Ru[P(OCH2)3CEt](NCMe)Ph][BAr'4] is known to selectively produce ethylbenzene, the formation of styrene using [(MeOTTM)Ru[P(OCH2)3CEt](NCMe)Ph][BAr'4] is attributed to the substituents on the triazole rings of the MeOTTM ligand.},
doi = {10.1002/ijch.201700099},
journal = {Israel Journal of Chemistry},
number = 10-11,
volume = 57,
place = {United States},
year = {2017},
month = {11}
}

Catalysts provide foundational technology for the development of new materials and can enhance the efficiency of routes to known materials. New catalyst technologies offer the possibility of reducing energy and raw material consumption as well as enabling chemical processes with a lower environmental impact. The rising demand and expense of fossil resources has strained national and global economies and has increased the importance of accessing more efficient catalytic processes for the conversion of hydrocarbons to useful products. The goals of the research are to develop and understand single-site homogeneous catalysts for the conversion of readily available hydrocarbons into useful materials.more » A detailed understanding of these catalytic reactions could lead to the development of catalysts with improved activity, longevity and selectivity. Such transformations could reduce the environmental impact of hydrocarbon functionalization, conserve energy and valuable fossil resources and provide new technologies for the production of liquid fuels. This project is a collaborative effort that incorporates both experimental and computational studies to understand the details of transition metal catalyzed C-H activation and C-C bond forming reactions with olefins. Accomplishments of the current funding period include: (1) We have completed and published studies of C-H activation and catalytic olefin hydroarylation by TpRu{l_brace}P(pyr){sub 3}{r_brace}(NCMe)R (pyr = N-pyrrolyl) complexes. While these systems efficiently initiate stoichiometric benzene C-H activation, catalytic olefin hydroarylation is hindered by inhibition of olefin coordination, which is a result of the steric bulk of the P(pyr){sub 3} ligand. (2) We have extended our studies of catalytic olefin hydroarylation by TpRu(L)(NCMe)Ph systems to L = P(OCH{sub 2}){sub 3}CEt. Thus, we have now completed detailed mechanistic studies of four systems with L = CO, PMe{sub 3}, P(pyr){sub 3} and P(OCH{sub 2}){sub 3}CEt, which has provided a comprehensive understanding of the impact of steric and electronic parameters of 'L' on the catalytic hydroarylation of olefins. (3) We have completed and published a detailed mechanistic study of stoichiometric aromatic C-H activation by TpRu(L)(NCMe)Ph (L = CO or PMe{sub 3}). These efforts have probed the impact of functionality para to the site of C-H activation for benzene substrates and have allowed us to develop a detailed model of the transition state for the C-H activation process. These results have led us to conclude that the C-H bond cleavage occurs by a {sigma}-bond metathesis process in which the C-H transfer is best viewed as an intramolecular proton transfer. (4) We have completed studies of Ru complexes possessing the N-heterocyclic carbene IMes (IMes = 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene). One of these systems is a unique four-coordinate Ru(II) complex that catalyzes the oxidative hydrophenylation of ethylene (in low yields) to produce styrene and ethane (utilizing ethylene as the hydrogen acceptor) as well as the hydrogenation of olefins, aldehydes and ketones. These results provide a map for the preparation of catalysts that are selective for oxidative olefin hydroarylation. (5) The ability of TpRu(PMe{sub 3})(NCMe)R systems to activate sp{sup 3} C-H bonds has been demonstrated including extension to subsequent C-C bond forming steps. These results open the door to the development of catalysts for the functionalization of more inert C-H bonds. (6) We have discovered that Pt(II) complexes supported by simple nitrogen-based ligands serve as catalysts for the hydroarylation of olefins. Given the extensive studies of Pt-based catalytic C-H activation, we believe these results will provide an entry point into an array of possible catalysts for hydrocarbon functionalization.« less

In this study, a series of Pt(II) complexes of the type ([(L~L)Pt(L')(Ph)][BAr' 4] (L~L = 1,2-bis(dimethylphosphino)ethane, 1,2-bis(diphenylphosphino)ethane, ( N-pyrrolyl) 2P(CH 2) 2P( N-pyrrolyl) 2, 1,3-bis(diphenylphosphino)propane, 1,1'-bis(diphenylphosphino)ferrocene, (bis-(diphenylphosphino)methyl)methylamine, 8-(diisopropylphosphino)quinoline, 1,1'-methylene-3,3'-di- tert-butylimidazol-2,2'-diylidine); L' = THF or NCMe) has been synthesized and fully characterized. These complexes were screened as catalysts for ethylene hydrophenylation to yield ethylbenzene. All of the complexes exhibited selectivity for styrene production with low catalytic turnover. DFT calculations have been used to model reactivity of the [(dmpe)Pt(L')(Ph)][BAr' 4] (dmpe = 1,2-bis(dimethylphosphino)ethane). It is shown that selective styrene formation is a result of a calculated ΔΔ G‡ of 5 kcal/molmore » for the benzene C–H activation step in the catalytic cycles for styrene versus ethylbenzene formation.« less

TpRu(CO)(NCMe)Ph is a catalyst for the conversion of benzene and ethylene to ethylbenzene. Previously, the formation of ethylbenzene has been shown to occur through a pathway that involves ethylene coordination to Ru, insertion of ethylene into the Ru–phenyl bond and Ru–mediated benzene C–H activation. Here, the effect of ethylene pressure and catalyst concentration (between 0.2 and 0.01 mol % based on benzene) on the decomposition of TpRu(CO)(NCMe)Ph was examined. Studies have shown that there are two competing catalyst deactivation pathways. At higher concentrations of TpRu(CO)(NCMe)Ph, the dominant deactivation pathway is likely initiated by a binuclear reaction of two Ru complexesmore » that leads to formation of unidentified paramagnetic species. Kinetic studies show that this pathway for catalyst decomposition occurs with a second-order rate of 0.007 (1) M -1 s -1. At lower Ru concentrations, ethylene C–H activation to form the allyl complex TpRu(CO)(η 3-C 4H 7) is the predominant deactivation pathway. The effect of ethylene pressure on catalyst decomposition was also examined. At higher ethylene pressure nearly quantitative formation of TpRu(CO)(η 3-C 4H 7) was observed.« less